1. Field of the Invention
The present invention relates to an image scanner, an auto document feeder, and an image forming apparatus.
2. Description of the Related Art
Conventionally, in order to correct, in an image scanner, influence on image data by a light source or a scanning optical system that illuminates a document, and determine a reference white level, there is generally performed shading correction using a scanned result (hereinafter referred to as shading data) of a density reference member having a given white color. The light amount of the light source decreases with time, and thus it is preferable to obtain shading data for each time performing image scanning.
However, when shading correction is performed for each time performing image scanning, the productivity is deteriorated because a period for generating shading data is required. In particular, when a white reference member scanning position is different from a document data scanning position in the sub-scanning direction, a process of moving a carriage is added, which significantly reduces production efficiency.
With such a background, there is proposed recently a technique to shorten paper interval time in a manner such that, at a document scanning position, not only a document but also a density reference member are scanned at the same time, the degree of decrease in light amount (correction factor) is calculated based on the comparison between the result of scanning the density reference member and the past result of scanning the density reference member, and the result of the correction using shading data is further corrected.
For example, Japanese Patent Application Laid-open No. 2006-13852 proposes a technique in which, in order to shorten shading data generation time in a paper interval, a light amount is corrected based on result of comparison among a plurality of positions of a facing member facing a scanned face and regarding a light amount reference value and a light amount variation that are obtained at the same position among the plurality of positions.
However, in the conventional configuration, when an image sensor (contact image sensor (CIS), for example) that does not include a pixel with its light-receiving portion shielded to detect output in darkness (optical black (OPB) pixel) is used, it is necessary to generate a reference black level after a light source is turned off once, thus making it difficult to significantly shorten paper interval time.
In an image sensor that includes no OPB pixel, it is necessary to turn off a light source once so as to regenerate reference black level data for black correction. Thus, the following two measures can be considered: (1) in order to shorten paper interval time, a light source is not turned off in each paper interval, and a black level generated when a job is started continues to be used; and (2) in order to maintain image quality, a light source is turned off in each paper interval or each specified number of paper intervals to regenerate a black level. However, these measures involve the following problems.
(1) In the case of a configuration in which a light source is not turned off in paper intervals:
A light source is continuously on until a job is finished, and thus heat generated by the light source increases a temperature of a sensor substrate. Consequently, due to difference in temperature characteristics between sensor chips, difference in an increased amount of a dark current (an increased amount of a reference black level) arises, which causes level difference between chips in a final scanned image.
(2) In the case of a configuration in which a light source is tuned off in a paper interval:
Even if one job takes long time and a temperature of a sensor substrate is increased, density difference between chips due to variation of a black level dose not arise in a scanned image when a reference black level is regenerated in each paper interval. However, this configuration requires turning off a light source; regenerating a black level; turning on a light source; and securing light amount stabilization time after a light source is turned on again. Therefore, there is a problem in time saving in this case. In particular, light amount stabilization time after a light source is turned on again is based on temperature characteristics of a light source. When the latest light source non-lighting time is long, a temperature of the light source is decreased, and thus necessary light amount stabilization time is increased (when the latest light source non-lighting time is short, by contrast, the decrease of a temperature of the light source is small, and thus light amount stabilization time after the light source is turned on again can be shortened).
In view of the above aspects, there is a need to shorten paper interval time while generating reference black data in a paper interval in an image scanner using an image sensor that does not include a pixel with light-receiving portion shielded (OPB pixels).